Powder mixtures having a cellulose ether binder are used in forming articles of various shapes. For example ceramic powder mixtures are formed into honeycombs which are used as substrates in catalytic and adsorption applications. The mixtures must be well blended and homogeneous in order for the resulting body to have good integrity in size and shape and uniform physical properties. The mixtures have organic additives in addition to the binders. These additives can be surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
A major and ongoing need in extrusion of bodies from highly filled powder mixtures, especially multicellular bodies such as honeycombs is to extrude a stiffer body without causing proportional increase in pressures. The need is becoming increasingly critical as thinner walled higher cell density cellular structures are becoming more in demand for various applications. Thin walled products with current technology are extremely difficult to handle without causing shape distortion.
Rapid-setting characteristics are important for honeycomb substrates. If the cell walls of the honeycomb can be solidified quickly after forming, the dimension of the greenware will not be altered in subsequent cutting and handling steps. This is especially true for a fragile thin-walled or complex shaped product, or a product having a large frontal area.
Prior rapid stiffening methods involve time-delayed stiffening using rapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652, and/or applying an external field such as an electrical, ultrasonic, or RF field at the die exit. All of these methods involve extrusion of soft batches. Historically, for highly filled ceramic mixtures, soft batches have lead to better extrusion quality. Attempts to extrude stiffer ceramic batches with the current batch components, i.e cellulose ether binder, lowering the amount of water and/or additives such as sodium tallowate or sodium stearate have not been very successful because of the higher extrusion pressures resulting from collision of finer particles, and the abrasiveness of the materials involved.
The growing need for thinner webs (1-2 mil)/high density cellular products to be extruded to shape necessitates stiffening at the very instant the batch exits the die.
More recently, the above problems have been solved by including more organic materials in the forming mixture such as disclosed in U.S. application Ser. Nos. 09/115,929, 09/116,144, and 60/095,292, among others. However, the organics can pose problems during the firing of the green bodies, due to exothermic reactions that can cause cracking in the bodies, resulting in a weakening structure. This especially true with multicellular structures such as honeycombs. In particular, very thin-walled structures are especially susceptible to cracking during firing.
The present invention fills the need for instantaneous forming of stiff batches which is especially beneficial for extrusion of thin walled honeycombs, and shape retention of extruded bodies at the very instant the batch exits the die, while at the same time providing for minimized exothermic reactions during the firing cycle to reduce the possibility of firing crack formation.